Longman Dictionary of Contemporary English
Wiktionary
n. (alternative form of whisper campaign English)
WordNet
n. the organized dissemination of derogatory rumors designed to discredit a candidate
Wikipedia
A whispering campaign or whisper campaign is a method of persuasion in which damaging rumors or innuendo are spread about the target, while the source of the rumors seeks to avoid being detected while spreading them (for example, a political campaign might distribute anonymous flyers attacking the other candidate). It is generally considered unethical in open societies, particularly in matters of public policy. The speed and anonymity of communication made possible by modern technologies like the Internet has increased public awareness of whisper campaigns and their ability to succeed. This phenomenon has also led to the failure of whisper campaigns, as those seeking to prevent them are able to publicize their existence much more readily than in the past. Whisper campaigns are defended in some circles as an efficient mechanism for underdogs who lack other resources to attack the powerful.
Usage examples of "whispering campaign".
It was now that a whispering campaign began to the effect that all southern slave masters were known to cohabit with slave women and that the Sage of Monticello was no exception.
Once, following a whispering campaign about whether he was a closet homosexual, because he hadn't fired a gay NSA employee, he felt it necessary to deny publicly that he was gay.
As for the whispering campaign concerning Bedwyour and the Queen, that was surely only a matter of policy, too.
For days and days, on and on, for dozens and then hundreds of soirees and balls and at homes, the whispering campaign pounded on.
There are lots of countries whose politicians were part of the F'thk whispering campaign.
I'm not even going to do anything about the whispering campaign he started.